The present invention relates to a technology of compressing image data. For example, the invention relates to a technology which enables shortening of a transfer time and simplification of a printer by compressing and transferring bit map data of a document to the printer when the document on a computer is printed and output by the printer.
As a conventional compression method of bit map images, there have been available Modified Huffman (MH), Modified READ (MR), Modified Modified READ (MMR), Joint Bi-Level Image Experts Group (JBIG), and the like, which have been widely used in FAX communication and image filing.
These compression methods are not suited to high-speed coding because it quite often uses bit processing. Thus, there have been presented a compression method and a compression device of bitmap data designed to achieve high-speed processing (Patent document 1).
According to the image compression method, as shown in FIG. 20(A), an input image 142 is scanned in a horizontal direction to separate an area 146 which includes a black pixel. The separated area is scanned in a vertical direction as shown in FIG. 20(B) to be separated into areas 152-1 to 152-3 which include black pixels.
An area 144-1 that includes no black pixel shown in FIG. 20(A) is coded as a skip 148 in the vertical direction, and areas 150-1, 150-2 that include no black pixels shown in FIG. 20(B) are coded as skips 151-1, 151-2 in the horizontal direction.
In the separated areas 152-1 to 152-3 that include the black pixels, identical column patterns 154 in which columns having equal pixel values continue are coded based on column pixel values and the number of repetitive times. The other column patterns are regarded as uncompressed areas, and pixel values thereof are directly coded and output.
Further, to increase compression ratios of the separated areas 152-1 to 152-3 which include the black pixels shown in FIG. 20(B), there have been presented a method of compressing a stepped pattern 156 in which column pixel values change in step fashion (Patent document 2), a method of compressing a repetitive pattern 158 in which patterns of certain columns and widths appear continuously and repeatedly (Patent document 3), and a method of compressing identical patterns which discretely appear (Patent document 4). Areas left after application of the image compression methods are directly coded and output as uncompressed areas 157-1 to 157-4 which cannot be compressed by adding type codeword indicating incompressibility to pixel values thereof.
[Patent document 1] Japanese Patent No. 3278298
[Patent document 2] Japanese Patent Application Laid-Open No. 2003-198855
[Patent document 3] Japanese Patent Application Laid-Open No. 2003-189108
[Patent document 4] Japanese Patent Application Laid-Open No. 2003-198856